Method and apparatus for cephalometric imaging

ABSTRACT

The invention relates to a method for cephalometric imaging using an apparatus ( 1 ) which comprises an X-ray source ( 5 ), a line detector camera ( 8 ) equipped with a digital detector, which line detector camera is located to a position far away from the X-ray source ( 5 ) for carrying out cephalometric imaging while the slot of the line detector camera is substantially vertical. In cephalometric imaging, an X-ray beam ( 11 ) located substantially on the vertical plane is directed from the X-ray source ( 5 ) through the object being imaged, and the X-ray source ( 5 ) is at the same time turned about the rotation center ( 13 ) located between the X-ray source and the line detector camera ( 8 ) in order to scan the object being imaged in the horizontal direction, whereby the line detector camera ( 8 ) is moved during the scanning movement in such a way that the ray beam ( 11 ) meets the slot ( 14 ) of the line detector camera. In the method, the effective focal spot is transferred from the rotation center ( 13 ) to the focal spot of the X-ray source ( 5 ) by transferring the rotation center by means of a transverse linear movement (A) with respect to the ray beam ( 11 ).

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. patentapplication Ser. No. 10/204,104, filed Oct. 10, 2002, now U.S. Pat. No.6,731,717. That application is the U.S. national stage application ofInternational Application PCT/FI01/00137, filed Feb. 14, 2001, whichapplication was published on Aug. 23, 2001 as International PublicationWO 01/60257. The International Application claims the priority ofFinnish Patent Application 20000369, filed Feb. 18, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to a method for cephalometric imagingusing an apparatus which comprises an X-ray source, a line detectorcamera equipped with a digital detector, which line detector camera islocated to a position far away from the X-ray source for carrying outcephalometric imaging while the slot of the line detector camera issubstantially vertical, whereby in cephalometric imaging, an X-ray beamlocated substantially on the vertical plane is directed from the X-raysource through the object being imaged, and the X-ray source is at thesame time turned about the rotation centre located between the X-raysource and the line detector camera in order to scan the object beingimaged in the horizontal direction, whereby the line detector camera ismoved during the scanning movement in such a way that the ray beam meetsthe slot of the line detector camera. The publication U.S. Pat. No.5,511,106 discloses this type of scanning method for cephalometricimaging.

SUMMARY OF THE INVENTION

When cephalometric imaging is carried out in the manner described above,the vertical ratio of enlargement remains the same as without thescanning movement, but the horizontal ratio of enlargement, on the otherhand, changes because the rotation centre changes into the effectivefocal spot instead of the focal spot of the X-ray source. Accordingly,the object of the present invention is to provide a method by means ofwhich the ratios of enlargement in cephalometric imaging can bemaintained the same in the vertical and horizontal directions incephalometric imaging, which is carried out by means of the turningmovement taking place about the rotation centre of the X-ray source. Afurther object is to provide a method which can be implemented by usingexisting panoramic X-ray imaging apparatuses, which already comprise themeans needed to perform the required movements. To achieve this object,the method relating to the invention is characterised in that in themethod, the effective focal spot is transferred from the rotation centreto the focal spot of the X-ray source by transferring the rotationcentre by means of a transverse linear movement with respect to the raybeam.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail in the following, withreference to the accompanying drawings, in which:

FIG. 1 shows a diagrammatic top view of an apparatus applicable for themethod according to the invention,

FIG. 2 shows diagrammatically a view in principle of a camera used inthe apparatus according to FIG. 1,

FIG. 3 shows diagrammatically the effect of the rotary movement of theX-ray source on the effective focus, and

FIG. 4 shows diagrammatically the effect of the transfer of the rotationcentre of the rotary movement of the X-ray source on the effectivefocus.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus 1 according to FIG. 1 comprises a support part 2, beneathwhich is connected a panoramic imaging apparatus 4 which turns in apivoting manner around the rotation centre 13, the imaging apparatuscomprising a C arm 7, on one vertical branch 7 a of which is arranged anX-ray source 5 and the primary collimator 6 in its vicinity, and on theother branch 7 b a line detector camera 8.

FIG. 2 shows diagrammatically one implementation of a line detectorcamera 8, wherein on one side of the camera is formed a slot 14, behindwhich, inside the camera 8, is a digital detector 15, for example, a CCDsensor. The panoramic imaging apparatus 4 and its operation are, assuch, known to a person skilled in the art. In panoramic imaging, theobject to be imaged is placed between the branch parts 7 a, 7 b of the Carm by means of appropriate guides and supports, after which the X-raysource 5 is switched on and the C arm is rotated about the rotationcentre 13, whereby the aperture of the primary collimator 6 is selectedin such a way that the ray beam 11, which is substantially on thevertical plane, is directed at the detector 15 behind the substantiallyvertical slot 14 of the line detector camera 8 on the branch part 7 b ofthe C arm 7, from which detector the image data is transmitted further,for example, to a microprocessor. This process of panoramic imaging willnot be described in greater detail in this connection.

To the support part 2 is connected a supporting arm 3, at the other endof which is the cephalometric imaging apparatus 10, which comprises aline detector camera 8 to be placed behind the object being imaged, anda secondary collimator 9 to be placed in front of the object beingimaged. In cephalometric imaging, the patient's head is scanned by meansof the ray beam 11 from right to left or vice versa.

In the method according to the invention, cephalometric imaging iscarried out by turning the X-ray source 5 about a rotation centre 13 toimage the patient's head 12 by means of a vertically positioned ray beam11. FIG. 3 shows a situation where the rotation centre 13 remains inplace, in which case, when the X-ray source 5 turns about the rotationcentre, the X-ray beams 11 travel in each angular position via therotation centre 13, whereby the rotation centre 13 becomes the effectivefocal spot for the radiation coming through the object in the layer 17being imaged on the image plane (detector) 18. In this case, the ratioof enlargement, which is determined by the ratio of the distance (SID)between the focus and the image plane to the distance (SOD) between thefocus and the object, is the ratio of the distance between the rotationcentre 13 and the image layer 17 to the distance between the rotationcentre 13 and the image plane 18.

It is for this reason that, according to the invention, in cephalometricimaging the rotation centre 13 is arranged to be transferred by linearmovement transversely with respect to the ray beam 11, as shown byreference marking A in FIG. 1. FIG. 4 shows a situation where therotation centre has been transferred by linear movement from position 13a to position 13 b, whereby the focal spot is transferred to the X-raysource 5, in which case also the horizontal ratio of enlargement remainsthe same.

1. A method for operating a panoramic imaging apparatus to provideimproved tomographic cephalometric images, the panoramic imagingapparatus being capable of operating in a mode in which an X-ray sourceand radiation sensor, spaced from said radiation source at a firstdistance, are rotated about an intermediate vertical rotation center toprovide a panoramic image from a subject positioned between the X-raysource and the radiation sensor, said method comprising the steps of:positioning a radiation receiver to receive radiation from said X-raysource, said radiation receiver being located at a second distance fromsaid X-ray source greater than said first distance; placing a subjectfor cephalometric imaging in front of the radiation receiver; projectingradiation from the X-ray source along a horizontal radiation path toapply radiation from the X-ray source to the radiation receiver;carrying out a first motion comprising rotating the X-ray source aboutthe center of rotation; carrying out a second motion comprisinghorizontally translating the center of rotation in a direction generallytransverse to the radiation path during application of the radiation tothe radiation receiver; and carrying out a third motion comprisingtranslating the radiation receiver in a horizontal direction generallytransverse to the radiation path, said first, second, and third motionsproducing a tomographic cephalometric image from the panoramic imagingapparatus for which the effective focal point for the X-ray source hasbeen altered in a manner to reduce differences in vertical andhorizontal ratios of enlargement.
 2. The method according to claim 1further including the step of collimating the radiation applied to theradiation receiver.
 3. An apparatus for providing a tomographiccephalometric image comprising: a radiation receiver; a panoramicimaging X-ray source providing radiation along a generally horizontallyextending radiation path for application to said radiation receiver; asupport for said X-ray source, said support having a vertical rotationcenter for rotating said X-ray source, said rotation center beingpositioned intermediate said radiation receiver and said X-ray sourceand closer to said X-ray source than to said radiation receiver; meansfor rotating said X-ray source about said rotation center; means forgenerally horizontally translating said rotation center in a directiongenerally transverse to the radiation path; and means for translatingsaid radiation receiver in a horizontal direction generally transverseto the radiation path during application of the radiation to saidradiation receiver, the motions provided to said radiation source,rotation center, and radiation receiver by the respective meansproducing a tomographic cephalometric image from a subject positioned infront of said radiation receiver for which the effective focal length ofthe X-ray source has been altered in a manner to reduce differences inthe vertical and horizontal ratios of enlargement.
 4. The apparatusaccording to claim 3 wherein said radiation receiver comprises a linedetection camera.
 5. The apparatus according to claim 3 furtherincluding a collimator for being placed in front of the subject.